The invention concerns a method for anaerobic treatment of wastewater and sludge in a septic tank in which the length of stay of the hydraulic components is decoupled from that of the particulate components in the septic tank, and a facility for implementing this method.
A method for wastewater purification is known from German Patent DE 10 2004 003 071 A1, in which the accumulating permeate is fed back for washing the digested sludge. Circulating accumulating sediments and using them further in paramount processes is known from German Patent DE 40 42 223 A1.
For the treatment of sludges (for example, primary or excess sludges from sewage treatment plants), manure of cattle, swine or chickens or even so-called nawaros (renewable resources), the facilities mentioned at the beginning are usually designed for a length of stay which comes to ca. 20-30 days with mesophile facilities and ca. 10-15 days with thermophile facilities. With such facilities, the length of stay of particulate components of sapropels (which can be designated as the anaerobic sludge age) in the container (which bring about the anaerobic process transformations and guarantees a process-stable and largely anaerobic digestion of the substrates) is almost identical with the length of stay of the hydraulic components. In particular, a high sludge ages guarantee the stable establishment of methanogenic bacteria populations which are responsible for the last and decisive process step of digestion, methane fermentation.
Such a facility operation, in which the length of stay of the particulate components is almost identical with the length of stay of the hydraulic components (in which thus no decoupling of the length of stay of the purely soluble phase from that of the particulate phase exists), usually takes place when the substrate to be treated is highly concentrated. This exists, for example, when sewage sludges with a TS concentration (TS=dry substance)>=5% TS (=50 g/l TS) are used or when fluid wastewater has an organic carbon component (CSB=chemical oxygen requirement) from which a TS concentration of most possibly >3% TS results after digestion.
The lower the substrate concentration is with this type of process engineering with completely mixed reactors without hydraulic decoupling, the more unfavorable is the profitability with regard to gas yield/m3 of reactor volume. Therefore sewage sludges with a TS concentration significantly below 5% dry substance are often subjected to a mechanical pre-thickening called concentrating to 5-6% TS.
Fluid wastewater with a low CSB concentration is in contrast often treated using special anaerobic processing engineering which guarantees a hydraulic decoupling. The widespread UASB method belongs to this. With this method, the liquid substrate to be digested is fed to a UASB reactor in the bottom region. The lower reactor region is filled with anaerobic granulas (that is anaerobic biology). When flowing through this region, the wastewater is digested and only the predominantly liquid digested phase enters through the upper reactor region into the outflow. In this way, the sapropel biomass is decoupled from the pure hydraulics, and the length of stay of this biomass is greatly increased in relation to the pure length of stay of the hydraulic components. The volume of the UASB reactor can consequently be designed smaller than according to usual purely hydraulic standards in connection with completely mixed reactors.
In both cases, the goal sought is nonetheless attained only on the basis of considerable technical additional expenditures. Moreover the anaerobic granulas prove to be unfavorable with regard to more extensive treatment methods, such as, for example, sapropel decomposition.